Provide platform_name in FCI L1C FDHSI reader.

[gerritholl]

In the FCI L1C FDHSI reader, provide the platform name as MTG-I1,
MTG-I2, MTG-I3, or MTG-I4, corresponding to the acronyms used by the WMO
OSCAR database. The reader translates those from the global NetCDF
attribute platform provided in each file, where it has the value
"MTI1" in the currently available test data.

• Closes The fci_l1c_fdhsi reader should proved the platform_name in the attributes #1014
• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[mraspaud]

LGTM. Indeed, we will see how the satellites are numbered after commissioning.

[coveralls]

Coverage increased (+0.006%) to 89.612% when pulling 512a3d8 on gerritholl:fci-platform-name into 65cd83f on pytroll:master.

[codecov]

Codecov Report

Merging #1176 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1176   +/-   ##
=======================================
  Coverage   89.60%   89.61%           
=======================================
  Files         200      200           
  Lines       29487    29504   +17     
=======================================
+ Hits        26422    26439   +17     
  Misses       3065     3065           

Impacted Files	Coverage Δ
satpy/readers/fci_l1c_fdhsi.py	96.21% <100.00%> (+0.05%)	⬆️
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py	100.00% <100.00%> (ø)

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[mraspaud]

Good question. I never really got the magic what happens on these sites.

[gerritholl]

@mraspaud I had deleted my comment to which you just replied because coveralls changed its mind and concluded the coverage increased (note the edit in the comment by coveralls).